GaN-based, high-mobility-electron transistors (GaN-HEMI) that use GaN in a channel layer, and an In-based nitride semiconductor in an electron supply layer (barrier layer) have been attracting attention. This structure enables to induce a high-concentration two-dimensional electron gas even when formed as a thinner layer, and hence, enables to obtain a superior high-output characteristic and a high-frequency characteristic compared with a structure that uses AlGaN in an electron supply layer.
However, an In-based nitride semiconductor has a problem in that the ohmic contact resistance is high. A high contact resistance reduces a maximum current between the source and the drain, and directly affects the output characteristic of an amplifier. Also, a higher contact resistance is more likely to induce an electric field concentration in the ohmic electrode, which expedites physical and chemical changes, and tends to lower the reliability of the ohmic electrode.
Regarding such reduction of the ohmic contact resistance, a structure has been publicly known in which a regrowth layer is formed to contain donors in an electron supply layer and a channel layer. This structure enables to reduce the ohmic contact resistance by providing the ohmic electrode over the regrowth layer.
However, use of such a regrowth layer containing donors introduces decline of the pinch-off characteristic, namely, increase of the drain leakage current, due to extensive and direct contact between the regrowth layer and donors that remain, for example, in the buffer layer and in a deep part of the channel located deeper than a region where the two-dimensional electron gas exists. As such, in a conventional semiconductor device, it is not possible to guarantee both the reduction of the ohmic contact resistance, and the superior pinch-off characteristic.